The present invention relates generally to switching power converters. More particularly, the present invention relates to DC-DC power converters configured with a plurality of stages for reducing voltage stress on circuit components and improving efficiency.
Flyback converter topologies are known in the art as low cost solutions for use in isolated power supplies. A flyback converter may provide power factor correction and isolated DC-DC power conversion at the same time. However, in applications having a wide range of potential input voltages, for example a 96V-312V input range, the voltage stress on components such as the main switch is sufficiently large that a switching component with a high voltage rating must be selected. This high voltage rating component, typically a MOSFET, is prohibitively expensive and can be difficult to obtain.
Referring to FIG. 1, one example of a conventional design for an offline flyback power converter 10 includes an input AC source V1 and a rectifier circuit including diodes D1-D4. An electromagnetic interference (EMI) filter may be provided with components L1, C1, L2 for reducing common-mode and differential-mode noise.
A main flyback transformer has a primary winding Tp and secondary winding Ts. A main switch Q1 is coupled in series with the primary winding Tp across positive and negative DC input terminals (i.e., as represented by V2 and GND_p, respectively). A control circuit CTL is used to drive the main switch Q1 to realize power factor correction and regulation of an output voltage V3.
The secondary winding Ts of the flyback transformer is coupled in series with an output rectifier D5 that converts energy from the secondary winding Ts to a DC voltage. An output capacitor C2 is coupled across the series circuit of Ts and D5 and supplies a load (R_load) for the particular application.
The voltage stress on the main switch Q1 when the switch Q1 is turned off is defined in accordance with equation (1):VQ1=√{square root over (2)}×V1_rms+N×Vout+Vleakage  (1)
If the input voltage V1 has a wide range, i.e. 108V-302V, and as additional parameters the turns ratio N=4 (as may typically be the case for low output voltage), Vleakage=70V and Vout=55V, a maximum voltage VQ1 across the main switch will be 717V. Therefore, a switch (e.g., MOSFET) Q1 with an 800V rating will have to be chosen to meet the voltage requirements of the system.
However, high voltage MOSFETs, such as for example those having an 800V rating, are expensive and difficult to obtain. Further, a typical on-resistance for a high voltage MOSFET is relatively high in comparison with a low voltage MOSFET that has the same current rating.
Briefly stated, if a conventional flyback topology is used for wide range input power factor correction (PFC) and DC-to-DC conversion applications, it is typically going to be prohibitively expensive and inefficient.